Adenocarcinoma of the prostate is the most common internal cancer of men in the United States and the second most frequent cause of cancer death (1). Identification of immunological and genetic reagents displaying specificity for prostate carcinoma versus normal prostate and BPH will be of immense value for the accurate diagnosis and possibly for the therapy of prostate cancer (2-6). Additionally, immunological reagents that can detect and bind to TAAs on the surface of prostate carcinoma cells, and that display limited expression on normal prostate and BPH tissue, may also prove of particular benefit for the detection and ultimately, therapy of prostate cancer (4,7,8).
Two novel approaches have been developed for defining the molecular and immunological basis of human cancer. One approach, the REXCS (2,3), permits the identification of putative oncogenic elements in human cancer DNAs. RExCS involves cotransfection of high molecular weight human tumor DNA in combination with a selectable antibiotic resistance gene into a new DNA acceptor cell line, CREF-Trans 6, selecting antibiotic resistant cells, injecting these cells into nude mice, and establishing cell cultures from tumors developing in athymic nude mice (2,3). Tumor-derived cell lines are then used to identify the putative human oncogenes, using approaches such as differential RNA display, that have been transferred from the human cancer cells into CREF-Trans 6 (3). Another methodology, SEM (7,8), results in the efficient and selective production of monoclonal antibodies reacting with tumor associated antigens present on the surface of human cancer cells (4,7,8). This procedure involves immunological subtraction based on the selective blocking of surface antigens on genetically altered cells (referred to as a "tester") with high titer polyclonal antibodies produced against untransfected parental cells (referred to as a "driver") This is followed by the production of hybridomas secreting MAbs that react with defined as well as uncharacterized cell surface expressed molecules (7,8).
Using SEM in combination with RExCS, prostate carcinoma (Pro 1.5) MAbs have been developed that react with TAAs expressed on human prostate cancer cell lines and patient-derived carcinomas (4,7,8). Addition of the Pro 1.5 MAb to nude mice containing human prostate cancer xenografts suppressed tumor growth and cancer progression. Screening a human LNCaP prostate cancer cDNA expression library with the Pro 1.5 MAb identified PCTA-1, a gene that encodes a novel galactose binding lectin, designated galectin-8HT (7). PCTA-1 was previously described in International Application No. PCT/US96/00307, filed Jan. 11, 1996, and published on Jul. 18, 1996 under International Publication No. WO 96/21671, which is herein incorporated by reference.
Using primer pairs corresponding to the 3' untranslated (UTR) region, RT-PCR detected PCTA-1 expression in prostatic carcinomas and PIN, but not in histologically confirmed normal prostate or BPH (7). Although further studies are necessary using a larger number of patient samples, these provocative results suggest that the Pro series of murine MAbs and the PCTA-1 gene should have direct diagnostic applications for human prostate cancer. Moreover, if the Pro MAbs show specificity toward prostate cancer, appropriately modified Pro MAbs, including chimerized and humanized versions of this MAb, may have therapeutic potential in patients with primary and metastatic prostate cancer.
PCTA-1 encodes a predicted protein structurally related to the S-type lectins (galectin) gene family (7). The galectins display wide tissue distribution, developmental regulation and differential expression levels in specific tissues, supporting the hypothesis that they contribute to many physiologically important processes in mammalian cells (9,10). Of direct relevance to cancer is the finding that the galectins, as well as the selectin subgroup of C-type lectins, mediate both cell-cell and cell-matrix interactions (10,11). These associations are critical elements in mediating the metastatic spread of tumor cells (12,13).